Alpha help!

i have a question.

mc instance name: box

when i apply this code to a movieclip
onClipEvent(load){
this.alpha = 0;
}
it works.

but

when this code applies
box.onEnterFrame = function(){
box._alpha = 0;
};
it wont work:(
any advice. i dont want to code on the main symbols, i want to put all the code in frames.